1. Field of the Invention
The present invention generally relates to semiconductor devices and more particularly to a method of fabricating a heterojunction bipolar transistor (HBT).
2. Description of the Related Art
HBT using a group III-V compound semiconductor material such as gallium arsenide (GaAs) is characterized by a high operational speed. This desirable operational characteristic is obtained mainly as a result of the high electron mobility in the group III-V compound forming the transistor. Further the use of a material having a large band gap for the emitter also contributes to the increase of the speed of the device.
In such an HBT, or in a bipolar transistor in general, it is desired that the base of the transistor is made as thin as possible in order to reduce the base transit time of electrons and hence to increase the operational speed of the transistor further. However, excessive reduction of the thickness of the base causes an adverse increase of the base spreading resistance that in turn decreases the speed of the transistor. Thus, in order to decrease the thickness of the base without increasing the base spreading resistance, it is desired to dope the base to a doping level as high as possible, for example to a level of about 1.times.10.sup.20 cm.sup.-3.
Meanwhile, it is known that the operational speed of the HBT is further improved by establishing an electric field in the base such that the electrons passing therethrough are accelerated. Such an electric field is obtained by providing a compositional gradient in the material forming the base such that there is a large band gap at a side of the base in contact with the emitter and such that the band gap is narrowed continuously toward the collector. For this purpose, gallium aluminum arsenide (GaAlAs) having a composition of Ga.sub.1-x Al.sub.x As is used for the base with a gradual change of the composition x ranging from 0 to 0.1. Note that there is substantially no change of lattice constant in the foregoing system and the problem of the lattice mismatch is avoided.
In an npn-type HBT constructed on a GaAs substrate with a base of GaAs doped to the p-type, beryllium (Be) is used as the dopant for the base. Unfortunately, it is known that the diffusion coefficient of Be in GaAs increases steeply by a factor of one thousand to even ten thousand when the concentration level of Be is increased beyond a level of about 4.times.10.sup.19 cm.sup.-3 (Pao, Y. C. et al. J. Appl. Phys. 60, 1986, p. 201). Further, it is known that such an increase of the diffusion coefficient of Be is further enhanced in the system of GaAlAs (Miller, D. L., J. Appl. Phys. 57, 1985, p. 1816). In other words, there is a tendency that Be is diffused into the neighboring collector and emitter during the fabrication process of the transistor, and there is a substantial risk that the conduction type of the emitter and collector is changed or the bipolar structure of the transistor is damaged. As a high temperature of about 600.degree. C. or more is needed for the growth of GaAlAs, the problem of diffusion of Be into the neighboring collector or emitter becomes worse in the case of the HBT using GaAlAs for the graded base. Thus, the conventional HBT has a problem in that it cannot be constructed to have a sufficiently reduced base thickness as well as a graded base profile to fully exploit the high speed performance of the HBT.
A similar problem appears also when the npn-type HBT is constructed on an indium phosphide (InP) substrate with a base of indium gallium aluminum arsenide (InGaAlAs) sandwiched by a collector and an emitter of indium gallium arsenide. Thus, when Be is doped to a level exceeding about 1.times.10.sup.20 cm.sup.-3 in InGaAlAs, the diffusion coefficient of Be is increased significantly.
On the other hand, there is a known HBT described by Wang et al. (Wang, K. C., Asbeck, P. M., Chang, M. F., Sullivan, G. J. and Miller, D. L., IEEE Electron Device Letters, EDL-8, no.9, September 1987, pp. 383-385) in which Be is doped to the base with a high concentration level together with an indium arsenide (InAs) component contained in the base by an amount of 10 mole %. In this device, the problem of excessive diffusion of Be is avoided by choosing InGaAs that is free from aluminum (Al) for the base so that the base can be grown in a low temperature range. This device, however, does not allow the compositional gradient in the base and thus the device has a limitation in the operational speed. Formation of the compositional gradation by adding elements other than Al to the system of InGaAs is generally not possible because of the unacceptable mismatch of lattice with respect to the collector or emitter.